1. Field of the Invention
This invention pertains to apparatus and method useful in developing information regarding geological formations and in particular in developing information in a deep marine environment utilizing a unique technique applied to seismic surveying that is based on the direction of the returned seismic reflections.
2. DESCRIPTION OF THE PRIOR ART
Marine seismic data acquisition techniques in current use are evolutionary techniques derived from techniques employed for developing 2-D marine seismic surveys. Such surveys are developed by a seismic vessel equipped with a seismic source and one or more seismic cables, sometimes referred to as streamers. A seismic cable is commonly on the order of two miles long and includes a plurality of pressure sensitive hydrophones spaced along its length. Individual hydrophone electrical outputs are often summed together forming arrays of the individual hydrophones. The arrays are often spaced at regular intervals, for example, every 15 meters, with the individual hydrophones also being regularly spaced. Conventionally a seismic source is positioned near the vessel and therefore also near the front end of the cable. The cable is ballasted to trail behind the vessel in a substantially horizontal plane and slightly beneath the surface of the water to avoid interference from surface wave action The receiving pattern for an array is the product of the element factor and the array factor The array factor has also been called the space factor It is the pattern of the array if the elements are isotropic. An individual hydrophone and its image (reflection off the sea's surface) form a dipole element. A typical hydrophone array will be more directional than a dipole (due to the array factor), but the array will still respond to returns over a wide angular range.
The seismic source is typically one or more airguns, waterguns, or marine vibrators which radiate wideband acoustic signals into the water at periodic intervals to be reflected from the geologic formations below the water bottom. In a 2-D survey, reflections from the formations are received by the arrays along the cable, routed to the vessel, and recorded for data processing. Each time the source radiates, a trace of seismic data is developed for each array. A seismic record is the total result of all the traces for a particular shot. Records of data are developed for an entire area of interest as the vessel traverses subsequent parallel paths at regular intervals, usually on the order of about one-half kilometer spacing.
3-D marine seismic surveys have been developed in a fashion similar to that described above for 2-D. However, there can be several receiver cables or streamers towed at once, possibly by more than one vessel. There are typically 240 arrays per cable and, therefore, for the two streamers shown in the prior art diagram shown in FIG. 1, for each shot occurrence there are 480 seismic traces. Since a shot occurs every few seconds, the resulting accumulation of seismic data from such a survey to be subsequently processed is enormous and extremely expensive. Furthermore, if the formations of survey interest are generally known through previous surveys or other information, much of the data collected by the above technique is noisy or complex. Note again from FIG. 1 that vessel 10 is off to one side of formation 12. The example source emanation 14 resulting in reflection 16 is useful. Subsurface lines 18 will contribute only when the vessel's track passes over structure 12. However, many of the other reflections that occur contemporaneously therewith do not reflect off the target of interest, formation 12. Nevertheless, the procedure utilized most frequently is the technique generally described above.
In antenna art it is known that transmitting antennas can be made to emit directional radiation. One such recognized technique that has been employed for this purpose is the "endfire" technique described for microwave antennas for example, in Microwave Scanning Antennas, Volume II, Array Theory and Practice, edited by R. C. Hansen, Academic Press, 1966, which is incorporated herein by reference. An endfire beam generally results from a linear array of individual elements. The array produces a tear-drop transmitting pattern from an end of the linear array.
The pattern's beamwidth is normally defined as the angular separation between directions at which the radiated power density is down to one-half its maximum value, and an endfire beam is very directional or narrow and is sometimes referred to as a "pencil beam" as opposed to a "fan beam" for a linear array at broadside. A "fan beam" only has a beamwidth for one angular coordinate, while a pencil beam is symmetrical about an axis along the array. Pencil beams whose half-power widths are in the region from about 15 degrees to 35 degrees can be produced quite readily by endfire arrays that have lengths ranging from 3-8 wavelengths. For endfire arrays the length of the array L varies inversely with the square of the beamwidth .theta..sub.B (.theta..sub.B.sup.2 .alpha.1/L) while the broadside beamwidth varies inversely with L.
Narrow pencil beams require very long arrays. By forming a composite array having individual endfire arrays as elements, a composite endfire beam can be made even more narrow.
Since the transmitting and receiving patterns of an array are identical through reciprocity, it is possible to build receiving arrays having the narrow pencil beams normally associated with endfire arrays
Therefore, it is a feature of the present invention to provide improved marine seismic data collection results utilizing seismic cable orientation that is generally vertical for pencil beam formation, rather than generally horizontal as employed in the prior art.
Another feature of the present invention is to provide improved seismic receiving arrays, in the form of "endfire" seismic receiving arrays covering the seismic frequency band, suitable for developing seismic data pertinent to a geologic formation having a generally predetermined location.
Another feature of this invention is to provide for the improved use of endfire arrays as the elements of an array comprised of endfire elements.
Yet another feature of this invention is to provide for the improved utilization of images above the ocean surface to effectively increase the length of the endfire array. The images can effectively double the length of a vertical cable.
Still another feature of the present invention is to provide improved direct placement of anticlinal structures in seismic data in a straightforward manner utilizing a pencil beam.